Water jet propulsion unit for use in water borne craft

ABSTRACT

A water jet propulsion unit suitable for use in outboard or stern-drive configuration in motor boats. The unit has an intake section, a pump section and a drive section within which is incorporated a pressure control device and nozzle. In the pump section there are two counter-rotating impellers on concentric counter-rotating shafts. The impellers are mounted upstream of the drive section. The drive section contains a drive transmission carrier to which is attached the pressure control device both being located in the vortex created in the water flow, downstream of the two impellers. The pressure control device allows the propulsion unit to operate in a low pressure high mass mode while maintaining pump priming. The impellers can be removed without disassembly of the drive transmission carrier.

TECHNICAL FIELD

The present invention relates to water jet propulsion units for use inwater borne craft.

BACKGROUND OF THE INVENTION

This invention describes a water jet propulsion unit but moreparticularly a fuel efficient unit which may be used in what arecommonly called outboard or stem-drive configurations in motor boats.The device utilises a pair of counter-rotating impellers and seeks tominimise internal frictional losses and the amount of kinetic energyimparted to the jet stream. It may also be described as an axial flowunit which achieves high efficiency gains by acting as a low pressurehigh mass device rather than a pressure inducing device.

The invention in general terms complies with the design and theoreticalrequirements described in our U.S. Pat. No. 5,634,831 but theconstraints imposed by the desirability of a compact and lightweightoutboard or stern-drive configuration introduces a set of parameterswhich do not apply to our previous designs. The device to be describedis thus a low pressure mass transfer device according to our previousinvention but with a drive transmission carrier and spring loadedpressure control device placed inside the flow vortex created downstreamof the two counter-rotating impellers. In our previous designs thecontrol devices are mounted to the perimeter walls of the nozzlesections. These control devices overcome the difficulties associatedwith the priming and pressure regulation of the low pressure high massdevice so described.

The constraints previously mentioned relate particularly to therequirement to have a right angle drive whereby it is necessary to placea driving shaft across the flow path of the water passing through theinside of the housing of the propulsion unit. Where this shaft passesthrough the flow path it is critical that it presents a hydrodynamicprofile to the impinging flow. The shaft is thus provided with anenclosing structure (vane) of the correct hydrodynamic shape. Thesection of the driving shaft which passes across the flow path must beof a minimal diameter in order to keep the support vane as narrow aspossible and yet have sufficient strength to enable it to reliablyaccept the load from the driving engine. In this case the enclosingstructure or vane is formed into the drive section and is also used tosupport the drive transmission carrier.

Because of the helical nature of the flow issuing from the upstreamimpeller it becomes critical, in order to maintain the efficiency of theunit, to also place support structures for the transmission drivecarrier in the correct position. ie: the vanes must preferably be placedin an area of axial or linear flow. The reason for this is that anyaxially aligned structure placed in a helically moving flow is actedupon by the whirl component of that flow. This results in a substantialincrease in the level of turbulence generated inside the unit andsubsequent interference with the helically moving flow impinging on thedownstream impeller. If axial flow is to be achieved at the nozzleoutlet any disturbance of the flow on the downstream impeller must beavoided in order for the counter-rotation cancellation effect to remainfully functional. These support structures are also required to have thecorrect hydrodynamic shape in order to maintain the efficiency of thedevice. Strict attention to potential frictional losses is particularlyimportant in a low pressure high mass device because through-pump flowvelocities are significantly higher than in conventional high pressurestatored designs absorbing the same horsepower.

Further, the efficiency of the unit is effected by the relative speed ofthe two impellers and so for the present invention the relative tipspeed of the impellers must be kept in the range of about 0 to 65 metersper second. Impeller tip speed, gearing and engine speed must all becalibrated according to this basic parameter.

In achieving a light weight design which allows ease of disassembly formaintenance purposes the positioning of the impellers in relation to theother mechanical components is also important. The present inventionallows this to be achieved without disassembly of the drive components,an important cost saving feature when propulsion units of this type arein routine commercial use. The primary reason for this being the need tomaintain impeller clearances inside the pump housing within acceptabletolerances.

In order for the driving components, such as the gears and bearings, tobe protected from dirt and other contaminants found in the marineenvironment, they must be enclosed in a lubricant filled water-tightcasing.

A further important constraint is the limited space afforded for thepositioning of bevel gears inside the drive transmission carrier whilstalso maintaining an axial flow configuration. The specific limitationapplies to the size of and therefore strength of the gears necessary forthem to accept the input power required. In the case of the stern-driveconfiguration we largely overcome this by providing for a geared speedreduction in a right angle drive external to the drive transmissioncarrier as described in the drawings. A one to one ratio in the bevelgears in the drive transmission carrier itself thus makes it possible tomaximise gear sizes and thus power input. Where size limitations in avery small construction adversely effects the overall design criteria,the pump casing walls may be diverged or coned slightly to providesufficient space for the transmission carrier and its components. Theshape of the impellers must be altered accordingly with due attention totheir relative speed and performance. In this case the upstream impelleris slightly conical in shape, at its periphery, with the downstreamimpeller also being conical but with approximately one third of thetrailing edge of its blades curving smoothly back to parallel with theaxis of the unit and the pump casing walls.

Examples of prior art constructions may be seen by reference to thefollowing; U.S. Pat. Nos. 3,082,732; 4,538996; and 4,872,858. In all ofthese devices the impeller is fixed to the vertically arranged shaft ofa conventional outboard layout. Water is drawn through the intake andcentrifugaly driven around a horizontally arranged bowl shaped pumphousing thence through a nozzle. The numerous directional changes andhigh pressure design limitations mean these devices allow high fuelconsumption and are generally less efficient than both conventionalinboard axial flow designs and propellered outboards. A further priorart construction being New Zealand 148,402 shows a pair of axialimpellers with stators in the nozzle section as described in New ZealandPatent Specification 123,228. In DE 39 42 672 Al are described severaldevices containing counter-rotating impellers two of which may be usedin outboard configuration where the input shaft passes through the flowinside the pump casing. The propulsion units described are of mixed flowdesign which the specification states is necessary because the increaseddiameter of a mixed flow impeller design over that of an axial designenables a drive transmission to be placed within two adjacent impellerhubs.

A number of fundamental design flaws mean that these devices can neverbe efficient. The reasons for this are as follows;

(i) None can function as a low pressure high mass device as described inour U.S. Pat. No. 5,634,831.

(ii) In the outboard designs described, the insertion of a drive shaftacross the flow path, between the two impellers, in a helically movingflow causes unacceptable losses.

(iii) The shape of the downstream impeller in all of the designsconforms to the shape of the nozzle portion of the pump housing , beingtapered or frusto conical in shape, where straightening vanes wouldnormally be placed in a conventional design. This part of the design isseriously flawed in that as water passes into the reduced nozzle area itwants to accelerate. However because the impeller blades (which are inthe nozzle) are progressively reducing in diameter, along the impellerhub, this means that the circumferential velocity of the impeller tipsis also reducing. As a result the amount of kinetic energy imparted tothe flow by the blades is also decreasing towards the downstream end ofthe impeller. The consequence of this is that the impeller blades try toslow the water down while the water itself wants to accelerate. Thiscauses further losses in the overall efficiency of these devices.

(iv) The exposure of the transmission driving components to the harshconditions found in the marine environment makes this design unsuitedfor commercial use.

(v) The insertion of a drive shaft between the impellers means that theincreased separation distance between the impellers increases in-pumpfrictional losses due to the fact that the flow between the impellers ishelical in motion. Ideally the impellers should be in closest proximityto each other in order to reduce the losses resulting from the waterhelically spinning inside the pump casing.

(vi) Between impeller insertion of the input shaft increases thecomplexity of the design in respect of maintenance.

The typical outboard-jet powered motor boat has a high fuel consumption.The primary reasons for this are that outboard jets currently in use donot conform well to propulsion theory in respect of kinetic energylosses to the jet stream and they have high internal frictional losseswhich arise from poor attention to the design and placement of internalpump structures.

OBJECTS OF THE INVENTION

It is therefore the principle object of the present invention to providean improved fuel efficient and compact water jet propulsion unit whichcan be used in either an outboard or stern-drive configuration.

The objectives and advantages of the present invention are accomplishedby ensuring that the unit operates in close accordance with propulsiontheory i.e. that the unit operates as a low pressure high mass deviceand that the design minimises internal frictional losses by correctplacement and design of the impellers and support vanes.

In addition, an upstream impeller is of a mixed flow configuration and adownstream impeller is of an axial flow configuration. Also a ratio of anozzle outlet area to that of a swept area of a forward impeller isabout 0.55 to 1.

The general design requirements are achieved by forwardly mounting theimpellers upstream of the drive transmission carrier. The importance ofthis arrangement being (i) achieving minimal distance between theimpellers (ii) correct placement of a support vane for through-flowplacement of the in-put shaft in an area of axial flow (iii) the abilityto place the pressure control device and drive transmission carrierinside the vortex created by the two impellers and (iv) ease ofmaintenance.

These and other objects and advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the followingdrawings herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional elevation of the stem-drive version of thejet propulsion unit showing the intake section in part view only.

FIG. 2 is a side sectional elevation of the outboard version of the jetpropulsion unit showing the intake section in part view only.

FIG. 3 is a cross section at A—A of the upper support vane in plan view.

FIG. 4 is an end elevation of the pressure control device showing thecircular arrangement of the over-lapping crown of flaps.

FIG. 5 is a side sectional elevation of the outboard version of the jetpropulsion unit.

FIG. 6 is a side sectional elevation of the outboard version of the jetpropulsion unit showing how the pump and drive sections can swing awayfrom the intake section.

FIG. 7 is a side sectional elevation of the outboard version of the jetpropulsion unit showing how the pump and drive sections can be separatedfrom the engine.

FIG. 8 is a side sectional elevation of the outboard version of the jetpropulsion unit showing how the drive section can be separated from thepump and intake sections.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 describes a stern drive configuration of an axial flow water jetpropulsion unit where an engine (not shown) may be coupled directly tothe drive flange 1. Counter-rotating impellers 2 and 3 are fixed toseparate concentric shafts 4 and 5 which in turn have spiral bevel gears6 and 7 fixed to their ends. These spiral bevel gears 6 and 7 meshjointly with the spiral bevel drive gear 8 which is fixed to thevertical shaft 9. Vertical shaft 9 has a spiral bevel gear 10 fixed toits end which is in turn driven by spiral bevel gear 11. Spiral bevelgear 11 is fixed to the prime input shaft 12 to which is connected thedrive flange 1. The spiral bevel gears 6 and 7 and the concentric shafts4 and 5 to which they are mounted are supported by needle rollerbearings 13 and 14 and by tapered roller bearings 15 and 16. Thrust fromimpeller 2 imposed on inner concentric shaft 5 is transferred from thedownstream spiral bevel gear 7, which is connected to concentric shaft5, to the inner face of spiral bevel gear 6 through an axial cylindricalroller bearing 17. The thrust generated by the downstream impeller 3fixed to the outer concentric shaft 4 is transferred to the taperedroller bearing 15. All the thrust generated by the impellers 2 and 3 forpropulsion is thus transferred to the tapered roller bearing 15. Seals18 and 19 serve to exclude water from the oil filled drive transmissioncarrier 20. The vertical shaft 9 passes through the centre of thesupport vane 21, FIG. 3, which joins the drive transmission carrier 20to the outer housing of the drive section 22. A similar vane 23 locatedon the opposite side of the drive transmission carrier 20 providesadditional locational support for the drive transmission carrier 20.These vanes 21 and 23 provide rigid support for the drive transmissioncarrier 20 and enable the thrust developed by the impellers 2 and 3 tobe transferred to the pump housing 24. Both vanes 21 and 23 arehydro-dynamically shaped in order to allow a smooth transition of flowthrough this portion of the jet propulsion unit and around the verticalshaft 9. Typically the vanes 21 and 23 have a length to width ratio ofabout four to one (4:1). The length of the vanes 21 and 23 may begreater but this increases the overall length of the jet propulsion unitmaking it less compact.

Thrust imposed on the vertical shaft 9 is taken by the bearing washer 25and needle rollers 26 and 27, the bearing washer 25 being sandwichedbetween the drive transmission carrier 20 and the base of the driveinput housing 28. The vertical shaft 9 is located by circlip 29.

For ease of assembly and maintenance, access to the drive transmissioncarrier 20 for insertion of spiral bevel gears 6,7 and 8 and bearings15,16,17 and 30 is achieved by providing a removable cone shaped bearingcarrier 31 at the upstream end of the drive transmission carrier 20 anda cover plate 32 at the downstream end.

The end cover plate 32 is fixed to the rear of the drive transmissioncarrier 20 by bolts 33 and 34. A pressure control device 35 is alsofixed to the rear of the drive transmission carrier 20. The pressurecontrol device 35 consists of an outer segmented ring or crown of flaps36 in close alignment, within which is found a further segmented ring orcrown of flaps 37, also in close alignment. The two sets of flaps 36 and37 evenly overlap one another so that when they expand they jointlyprovide a continuous surface to the approaching water flow. FIG. 4 showsan end view which indicates the manner in which the flaps 36 and 37 arearranged in respect to each other. A centrally mounted shaft 38 formedas part of the inner flap set 37 provides location for a tapered andcircular plunger cone 39 which is able to move up and down the shaft 38.FIGS. 1, 2 and 4 show the two sets of flaps 36 and 37 in the fullyretracted position. A spring 40, retained by the seat 41 and nut 42provides a means of tensioning the plunger cone 39 which in turn movesdown the shaft 38 thereby pushing out both sets of flaps 36 and 37 sothat they reduce the effective outlet area of the nozzle 43. Under lowflow conditions, at start-up, this thus enables the unit to prime andalso retain low pressure high mass conditions as the impellers 2 and 3speed up. As the pressure increases the flaps 36 and 37 of the pressurecontrol device 35 automatically close inwards allowing the outlet areaof the nozzle 43 to increase, thereby maintaining a constantly reducedpressure regime within the jet propulsion unit and therefore a high masstransfer rate. The spring 40 provides sufficient tension for backpressure conditions within the unit to be retained so that the impellers2 and 3 remain loaded at start-up and in the acceleration phase. Thepressure control device 35 thus provides a constantly variable means ofinstantaneously controlling the internal pressure of the unit in boththe acceleration and deceleration phase whilst also allowing priming tooccur.

FIG. 2 shows a substantially similar jet propulsion device but with thedrive input housing removed allowing the vertical shaft 9 to be coupleddirectly to an engine 63 in a typical outboard configuration.

FIGS. 5 to 8 show the manner in which an engine may be attachedvertically to the shaft 9 in outboard configuration.

Detachment of the pump section 44 is achieved by first of all removingthe nut(s) 45 which in turn permits the entire outboard unit 46 to beswiveled back away from the transom 47 as shown in FIGS. 5 and 6. Ahinge point 48 seen in FIGS. 5 to 8 allows this to be achieved and the Oring seal 49 shown in FIGS. 5 to 8 can be removed. Removal of nuts 50and 51 then allows the entire pump section 44, less intake section 52 ,to be removed as shown in FIG. 7. Disassembly of the pump section 44 canthen be accomplished by removing the pump housing flange 53 and drivetransmission carrier flange 54 fixing bolts and nuts (not shown),permitting the withdrawal of the impellers 2 and 3 and drivetransmission carrier 20, as a single assembly from the pump housing 24,as shown in FIG. 8. The impellers 2 and 3 can then be removed byunscrewing the retaining screw 55 and the retaining nut 56, both seen inFIG. 1. The inner wall of the pump housing 24 is slightly tapered sothat the impeller blades 57 and 58 which have matching tapers, are ableto be moved forward progressively as they wear so that the correctclearances are maintained. This is accomplished by placing spacerwashers (not shown) behind the impeller seats 59 and 60. This thusgreatly increases the useful life of the impellers 2 and 3 before buildup of the impeller tips 61 and 62 is required. The impeller blades 57and 58 typically have blade angles in the range of about 30 to 40degrees in order to maximise the flow rate needed for efficient thrustout-put. Further improvements arise by ensuring that the blades 57 and58 are of thinnest practicable section and have an aerofoiled profile.

What is claimed is:
 1. A water jet propulsion unit comprising: an intakesection; an axial flow pump section; a drive section, all in smoothcommunication with one another; said drive section containing a drivetransmission carrier and pressure control device; said pressure controldevice being mounted on said drive transmission carrier, downstream ofsaid drive transmission carrier; said drive transmission carrierconsisting of a removable upstream mounted cone shaped bearing carrier,gears and downstream mounted cover plate; said pump section containing asingle pair of counter-rotating axial flow impellers; said impellersbeing each mounted on separate concentric counter-rotating shaftsextending forwardly from said drive transmission carrier; said drivetransmission carrier being located in said drive section byhydrodynamically shaped support vanes downstream of said axial flowimpellers; one of said support vanes having a vertically aligned hole topermit a vertically aligned input drive shaft to connect to said drivetransmission carrier; said drive section being detachable from said pumpsection and said intake section; said pressure control device permittingthe unit to operate in a low pressure high mass mode; said impellersbeing removable without disassembly of said drive section; saidpropulsion unit being mountable in either a stern-drive or outboardconfiguration.
 2. The propulsion unit as claimed in claim 1 where thereare no support structures forward or between the two impellers.
 3. Thepropulsion unit as claimed in claim 1 where the impellers are mountedupstream of the drive section.
 4. The propulsion unit as claimed inclaim 1 where the coaxial drive shafts extend upstream from said gearsin said drive transmission section.
 5. The propulsion unit as claimed inclaim 1 which is used in a stern-drive configuration.
 6. The propulsionunit as claimed in claim 1, which is used in an outboard configuration.7. The propulsion device a claimed in claim 1 where the ratio of thenozzle outlet area to that of the swept area of the forward impellerbeing about 0.55 to
 1. 8. A water jet propulsion unit comprising: anintake section; an axial flow pump section; a drive section, all insmooth communication with one another; said drive section containing adrive transmission carrier and pressure control device; said pressurecontrol device being mounted on said drive transmission carrier,downstream of said drive transmission carrier; said drive transmissioncarrier consisting of a removable upstream mounted cone shaped bearingcarrier, gears and downstream mounted cover plate; said pump sectioncontaining a single pair of counter-rotating axial flow impellers; saidimpellers being each mounted on separate concentric counter-rotatingshafts extending forwardly from said drive transmission carrier, saiddrive transmission carrier being located in said drive section byhydrodynamically shaped support vanes downstream of said axial flowimpellers; one of said support vanes having a vertically aligned hole topermit a vertically aligned input drive shaft to connect to said drivetransmission carrier; said drive section being detachable from said pumpsection and said intake section; said pressure control device permittingthe unit to operate in a low pressure high mass mode; said impellersbeing removable without disassembly of said drive section; saidpropulsion unit being mountable in either a stern-drive or outboardconfiguration, the drive transmission carrier and the pressure controldevice being mounted in a vortex created in the water flow by the twoimpellers.
 9. A water jet propulsion unit comprising: an intake section;an axial flow pump section; a drive section, all in smooth communicationwith one another; said drive section containing a drive transmissioncarrier and pressure control device; said pressure control device beingmounted on said drive transmission carrier, downstream of said drivetransmission carrier; said drive transmission carrier consisting of aremovable upstream mounted cone shaped bearing carrier, gears anddownstream mounted cover plate; said pump section containing a singlepair of counter-rotating axial flow impellers; said impellers being eachmounted on separate concentric counter-rotating shafts extendingforwardly from said drive transmission carrier, said drive transmissioncarrier being located in said drive section by hydrodynamically shapedsupport vanes downstream of said axial flow impellers; one of saidsupport vanes having a vertically aligned hole to permit a verticallyaligned input drive shaft to connect to said drive transmission carrier;said drive section being detachable from said pump section and saidintake section; said pressure control device permitting the unit tooperate in a low pressure high mass mode; said impellers being removablewithout disassembly of said drive section; said propulsion unit beingmountable in either a stern-drive or outboard configuration, the onesupport vane for the drive transmission carrier having a length to widthratio of about 4 to
 1. 10. A water jet propulsion unit comprising: anintake section; an axial flow pump section; a drive section, all insmooth communication with one another; said drive section containing adrive transmission carrier and pressure control device; said pressurecontrol device being mounted on said drive transmission carrier,downstream of said drive transmission carrier; said drive transmissioncarrier consisting of a removable upstream mounted cone shaped bearingcarrier, gears and downstream mounted cover plate; said pump sectioncontaining a single pair of counter-rotating axial flow impellers; saidimpellers being each mounted on separate concentric counter-rotatingshafts extending forwardly from said drive transmission carrier, saiddrive transmission carrier being located in said drive section byhydrodynamically shaped support vanes downstream of said axial flowimpellers; one of said support vanes having a vertically aligned hole topermit a vertically aligned input drive shaft to connect to said drivetransmission carrier; said drive section being detachable from said pumpsection and said intake section; said pressure control device permittingthe unit to operate in a low pressure high mass mode; said impellersbeing removable without disassembly of said drive section; saidpropulsion unit being mountable in either a stern-drive or outboardconfiguration, the tip speed between the two axial flow impellers beingset in the range of about 0 to 65 meters per second.
 11. A water jetpropulsion unit comprising: an intake section; an axial flow pumpsection; a drive section, all in smooth communication with one another;said drive section containing a drive transmission carrier and pressurecontrol device; said pressure control device being mounted on said drivetransmission carrier, downstream of said drive transmission carrier;said drive transmission carrier consisting of a removable upstreammounted cone shaped bearing carrier, gears and downstream mounted coverplate; said pump section containing a single pair of counter-rotatingaxial flow impellers; said impellers being each mounted on separateconcentric counter-rotating shafts extending forwardly from said drivetransmission carrier, said drive transmission carrier being located insaid drive section by hydrodynamically shaped support vanes downstreamof said axial flow impellers; one of said support vanes having avertically aligned hole to permit a vertically aligned input drive shaftto connect to said drive transmission carrier; said drive section beingdetachable from said pump section and said intake section; said pressurecontrol device permitting the unit to operate in a low pressure highmass mode; said impellers being removable without disassembly of saiddrive section; said propulsion unit being mountable in either astern-drive or outboard configuration, said axial flow impellers havingperipheral blade angles in the range of about 30 to 40 degrees.
 12. Awater jet propulsion unit comprising: an intake section; an axial flowpump section; a drive section, all in smooth communication with oneanother; said drive section containing a drive transmission carrier andpressure control device; said pressure control device being mounted onsaid drive transmission carrier, downstream of said drive transmissioncarrier; said drive transmission carrier consisting of a removableupstream mounted cone shaped bearing carrier, gears and downstreammounted cover plate; said pump section containing a single pair ofcounter-rotating axial flow impellers; said impellers being each mountedon separate concentric counter-rotating shafts extending forwardly fromsaid drive transmission carrier, said drive transmission carrier beinglocated in said drive section by hydrodynamically shaped support vanesdownstream of said axial flow impellers; one of said support vaneshaving a vertically aligned hole to permit a vertically aligned inputdrive shaft to connect to said drive transmission carrier; said drivesection being detachable from said pump section and said intake section;said pressure control device permitting the unit to operate in a lowpressure high mass mode; said impellers being removable withoutdisassembly of said drive section; said propulsion unit being mountablein either a stern-drive or outboard configuration, said pressure controldevice comprises two layers of over-lapping flaps in circulararrangement being tensioned by a cone and spring.
 13. A water jetpropulsion unit comprising: an intake section; an axial flow pumpsection; a drive section, all in smooth communication with one another;said drive section containing a drive transmission carrier and pressurecontrol device; said pressure control device being mounted on said drivetransmission carrier, downstream of said drive transmission carrier,said drive transmission carrier consisting of a removable upstreammounted cone shaped bearing carrier, gears and downstream mounted coverplate; said pump section containing a single pair of counter-rotatingaxial and mixed flow impellers; an upstream one of said pair ofimpellers being of said mixed flow configuration and a downstream one ofsaid pair of impellers being of said axial flow configuration; saidimpellers being each mounted on separate concentric counter-rotatingshafts extending forwardly from said drive transmission carrier; saiddrive transmission carrier being located in said drive section byhydrodynamically shaped support vanes downstream of said axial flowimpellers; one of said support vanes having a vertically aligned hole topermit a vertically aligned input drive shaft to connect to said drivetransmission carrier; said drive section being detachable from said pumpsection and said intake section; said pressure control device permittingthe unit to operate in a low pressure high mass mode; said impellersbeing removable without disassembly of said drive section; saidpropulsion unit being mountable in either a stern-drive or outboardconfiguration.
 14. A water jet propulsion unit comprising: an intakesection; an axial flow pump section; a drive section, all in smoothcommunication with one another; said drive section containing a drivetransmission carrier and pressure control device; said pressure controldevice being mounted on said drive transmission carrier, downstream ofsaid drive transmission carrier; said drive transmission carrierconsisting of a removable upstream mounted cone shaped bearing carrier,gears and downstream mounted cover plate; said pump section containing asingle pair of counter-rotating axial flow impellers; said impellersbeing each mounted on separate concentric counter-rotating shaftsextending forwardly from said drive transmission carrier, said drivetransmission carrier being located in said drive section byhydrodynamically shaped support vanes downstream of said axial flowimpellers; one of said support vanes having a vertically aligned hole topermit a vertically aligned input drive shaft to connect to said drivetransmission carrier; said drive section being detachable from said pumpsection and said intake section; said pressure control device permittingthe unit to operate in a low pressure high mass mode; said impellersbeing removable without disassembly of said drive section; saidpropulsion unit being mountable in either a stern-drive or outboardconfiguration, the drive section and pump housing being swivelled awayfrom the intake section.